1. Field of the Invention
The present invention relates, in general, to a method for forming tungsten plugs in contact holes of a semiconductor device, and more particularly, to reducing contact resistance and maintaining control for the heights of the plugs.
2. Description of the Prior Art
Generally, a method for fabricating a semiconductor device comprises forming a contact hole as a connecting portion on a substrate through an insulating film and burying a suitable metal, for example, tungsten in the contact hole to electrically connect the substrate with a metal layer positioned above the substrate. When such connecting portion is formed, there are generated problems as follows: first, resistance is increased between the plug formed in the contact hole and the metal layer; and second, in the case that the plug is formed by at least 2-step processes in a contact hole having nonuniform depth, the height that the plug, for example, a tungsten plug grows up to cannot be controlled appropriately.
In efforts to solve the aforementioned problems, there have been suggested many methods. For example, Japanese Pat. Laid-Open Publication No. Heisei 3-262118 to Sakamoto, Laid-Open Publication No. Heisei 4-65129 to Tamura, Laid-Open Publication Nos. Heisei 4-142061 and 4-336422 to Hasegawa, Laid-Open Publication No. Heisei 4-7823 to Keoda, Laid-Open Publication No. Heisei 4-291746 to Shinohara, Laid-Open Publication No. Heisei 4-142062 to Nakatani, Laid-Open Publication No. Heisei 4-278526 to Ishihara, Laid-Open Publication No. Heisei 4-286325 to Watanabe, et al, Laid-Open publication No. Heisei 4-294531 to Muroyama, Laid-Open publication No. Heisei 4-307933 to Saito, et al, Laid-Open Publication No. Heisei 4-307934 to Saito and Laid-Open Publication No. Heisei 5-3171 to Miyamoto, and U.S. Pat. No. 5,202,579 to Fujii, et. al., U.S. Pat. Nos. 5,183,782 to Onishi, et. al. and 4,822,753 to Pintchovsky, et. al. have disclosed methods for reducing the contact resistance of tungsten plugs, for improving the wire reliability by the planarization of the upper surface of the tungsten plugs grown, and for improving the step coverage of the metal layer formed uppermost by taping the edge of the contact hole.
However, with regard to the adjustment of the heights of the tungsten plugs when the tungsten plugs are formed by the combination of different steps in the contact holes having nonuniform depths, the methods proposed from the above patents have not been completely successful and the contact resistance cannot be reduced thereby.
Hereinafter, description of the prior art for forming tungsten plugs along with the problems generated therefrom will be given for the better understanding, with reference to FIG. 1A through 1D, wherein are shown process steps for forming tungsten plugs in contact holes with different depths.
Referring initially to FIG. 1A, there is a schematic, cross sectional view illustrating the step of forming a contact hole. For this, a substrate 1 is first sectioned into an active region and a device separation region by a field oxide film 2 and is entirely covered with a first insulating layer 3. Over a predetermined portion of the first insulating film 3 atop the field oxide film 2 is formed a conductive wire 4 and then, over the resulting structure is coated a second insulating layer 5, which is subsequently covered with a first photosensitive pattern 6 for contact mask. Using the first photosensitive pattern 6, an etch process is applied to the first and the second insulating films 3 and 5 to form a first contact hole 7 on the active region of the substrate 1. The dot lines shown in this figure represent the region which is to be provided to form another contact hole.
Referring now to FIG. 1B, there is shown a first tungsten plug 8 of predermined thickness which is buried in the first contact hole 7 after removing the first photosensitive pattern 6 and of which upper part is placed under the surface of the second insulating layer 5. The first tungsten plug 8 formed is so tall as to make the first contact hole 7 have the same or similar depth as a second contact hole, represented by dot lines, to be formed later.
Turning now to FIG. 1C, there is illustrated an etching process which is applied to the second insulating layer 5 exposed after forming a second photosensitive film pattern 9 for contact mask for the purpose of forming a second contact hole 10 exposing the conductive wire 4 therethrough.
Finally, with reference to FIG. 1D, there is a schematic, cross sectional view illustrating the step of connecting with a metal layer. For this, after removing the second photosensitive pattern 9, the resulting first contact hole 7 and the second contact hole 10 are filled with tungsten plugs 11 and then, a metal layer 12 is formed on the entire resulting structure. As a result, the metal layer 12 is connected with the active region of the substrate 1 through the first and the second tungsten plugs and with the conductive wire 4 through the second tungsten plug.
Since the first tungsten plug is deposited on the substrate of the first contact hole at a thickness corresponding to approximately half depth of the contact hole and the second tungsten plug is deposited over the first tungsten plug according to the described conventional method, the thickness of the first tungsten plug should be carefully decided in consideration of second contact hole depth. That is, unless the upper surface of the first tungsten plug 8 reaches the same height with the upper surface of the conductive wire 4, the metal layer 12 is formed badly since the second tungsten plug 11 is uniformly thick. What is worse, the patterning process of the second photosensitive film for contact mask may contaminate the upper surface of the first tungsten plug, to increase the contact resistance between the first tungsten plug and the second tungsten plug in the first contact hole.